Build package.xml for Metadata Records

MD.apxc

public class MD {
	public static String buildPackageXMLForMD(SObjectType mdt, String whereClause){
        String mdtName = [ select DeveloperName FROM EntityDefinition where QualifiedAPIName = :mdt.getDescribe().getName() ].DeveloperName;
        
        List<SObject> records = Database.query('SELECT DeveloperName FROM ' + mdt + whereClause );
        List<String> items = new List<String>();
        for (SObject r: records) {
            items.add( mdtName + '.' + r.get( 'DeveloperName' )  );
        }
        return '    <types>\r\n        <members>' + String.join( items, '</members>\r\n        <members>' ) + '</members>\r\n        <name>CustomMetadata</name>\r\n    </types>';
    }
    
    public static void sendPackageXMLEmail(String attachmentContent) {
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        mail.setToAddresses(new List<String>{UserInfo.getUserId()});
        mail.setSubject('Package.xml prepared');
        mail.setHtmlBody('');
        if (attachmentContent != null) {
            Messaging.EmailFileAttachment attachment = new Messaging.EmailFileAttachment();
            attachment.setFileName('package.xml');
            attachment.setBody(Blob.valueOf( attachmentContent));
            mail.setFileAttachments(new Messaging.EmailFileAttachment[]{
                attachment
            });
        }
        Messaging.sendEmail(new Messaging.SingleEmailMessage[]{
            mail
        });
    }
}